The present invention relates to a method of forming a coupled coil arrangement of the type that, for example, is used on a superconducting coil in a superconducting magnet. The present invention also relates to a superconducting solder of the type that, for example, is used to couple a lead of a first coil to a lead of a second coil and also to couple a lead from a section of one coil to a lead of another section within the same coil.
In the field of Magnetic Resonance Imaging (MRI), it is known to provide a superconducting magnet in order to generate a strong uniform static magnetic field, known as a B0 field, in order to polarise nuclear spins in an object under test.
The superconductive magnet typically comprises a coil support structure carrying windings of a superconducting wire formed from an alloy that exhibits the property of superconduction at very low temperatures. In this respect, a number of coils are formed about the coil support structure and it is necessary to connect the coils in series. Each coil has a respective pair of leads, the leads being used, inter alia, to couple the coils in series. For some superconductive magnet designs, one or more of the coils can comprise a number of coil sections joined together in series to form a given coil.
Prior to series connection of the coil sections and/or coils, the coils are encapsulated in resin to prevent movement of the windings. However, after encapsulation of the coils in resin, the leads need to be cleaned in order to remove residual resin from the leads. This cleaning process can result in the leads becoming damaged, thereby causing whole coils or coil sections to become waste. It is therefore desirable to connect the leads and hence coils prior to encapsulation of the coil sections and/or coils in resin. In this respect, a common process used to joint coil sections and/or coils employs a superconductive alloy known as “Wood's metal” to joint two leads from respective coils or coil sections as part of the series coupling. However, due to the relatively low melting point of the Wood's metal, the resin encapsulation process will cause any solder joints formed using the Wood's metal to melt, thereby reducing the integrity of the solder joints. Consequently, soldering of joints takes place after the encapsulation stage and the leads are not encapsulated in the resin. Instead, the encapsulated leads are restrained to avoid movement thereof during operation of the superconducting magnet comprising the coils. Additionally, the Wood's metal contains cadmium, which is environmentally hazardous and so requires special precautions during use to protect the health of those using the Wood's metal.
According to a first aspect of the present invention, there is provided a method of forming an encapsulated coupled coil arrangement. The method comprises: coupling a first lead of a first coil to a second lead of an electrical circuit device, by soldering or infusion, using a superconductive jointing alloy; and encapsulating the first coil, the electrical circuit device and the jointed leads of the first coil and the electrical circuit device in an encapsulation material. The jointing alloy has a melting point higher than a highest temperature experienced by the encapsulation material during the encapsulation process.
For the avoidance of doubt, references herein to coils, coil sections and switches are examples of electrical circuit devices.
The coupling of the first lead of the first coil and the second lead of the electrical circuit device may result in the first coil and electrical circuit device being series coupled.
The superconductive jointing alloy may be a superconductor at low temperatures. The superconductive jointing alloy may have a superconducting transition temperature below about 10 Kelvin. The superconducting transition temperature may be below about 9.2 Kelvin. The superconducting transition temperature may be below about 5 Kelvin, for example about 4.2 K.
The electrical circuit device may be a second coil.
The electrical circuit device may be a cryogenic switch.
The superconductive jointing alloy may comprise: from 40 to 56 wt. % bismuth, and from 44 to 60 wt. % lead, and a balance of unavoidable impurities.
The superconductive jointing alloy may comprise: from 30 to 55 wt. % bismuth, from 30 to 50 wt. % lead, and from 15 to 30 wt. % tin, and a balance of unavoidable impurities.
The superconductive jointing alloy may comprise from 35 to 45 wt. % bismuth. The superconductive jointing alloy may comprise from 35 to 45 wt. % lead. The superconductive jointing alloy may comprise from 17 to 20 wt. % tin.
The superconductive jointing alloy may comprise approximately 55.5 wt. % Bi and approximately 44.5 wt. % Pb.
The superconductive jointing alloy may comprise approximately 52.5 wt. % Bi, approximately 32 wt. % Pb and approximately 15.5 wt. % Sn.
The superconductive alloy may be in the form of a bar, a stick, a powder, a solid or cored wire, a foil, a preform or a paste.
The superconductive alloy may have a melting point above about 90° C.
The superconductive alloy may have a melting point of about 95° C. The superconductive alloy may have a melting point above about 96° C.
The superconductive alloy may have a melting point below about 137° C.
The encapsulation material may be a thermoset material. The thermoset material may be a resin.
The encapsulation material may be a thermoplastic material. The thermoplastic material may be wax or polyester.
According to a second aspect of the present invention, there is provided a method of forming a superconductive magnet for tomography, the method comprising the method of forming a coupled coil arrangement as set forth above in relation to the first aspect of the invention.
It is thus possible to provide a method of coupling a lead of a first coil to a lead of a second coil that enables the coils including the coupled leads to be encapsulated without damage to a solder joint coupling the leads. Consequently, the leads do not have to undergo a cleaning stage and do not have to be restrained so as to avoid movement of the leads in an active electromagnetic field. The use of the superconductive jointing alloy is advantageously compatible with existing manufacturing processes for the production of the superconducting magnet and obviates the use of Cadmium.